The Assignee of the present invention manufactures electrical power generating systems for airframes which convert a variable speed shaft output from a propulsion engine into a constant speed output drive which drives a three phase alternator to produce 400 Hz. electrical power. These systems are known as integrated drive generators. Integrated drive generators require a minimum fluid flow of oil for proper operation. Operation of an integrated drive generator with a supply of oil below the minimum flow for a significant time interval can cause catastrophic failure or serious damage.
FIG. 1 illustrates a prior art diagram showing the oil circulation circuit of an integrated drive generator of the type manufactured by the Assignee, such as used in the Assignee's model 75EGS01I integrated drive generator. The oil circulation system 10 includes an integrated drive generator 12 having an oil input 14 and an oil output 16. The construction and operation of integrated drive generators are well known and described in numerous patents of the Assignee as is not part of the present invention. The output oil from output 16 of the integrated drive generator 12 flows to scavenge pump 18 through input 20. The oil flow 62 from output 22 of the scavenge pump 18 flows to a recirculation valve 23. Additionally, an aircraft mounted accessory drive 24 receives oil flowing from the recirculation valve 23 at an input 26. The aircraft mounted accessory drive 24 supplies shaft power to the integrated drive generator 12 in a well-known manner. A supply pump 28 is located within an oil reservoir 30 of the aircraft mounted accessory drive for pumping oil flow 60 to the recirculation valve 23.
The recirculation valve 23 is comprised of a first input 32 and a second input 34 which respectively receive oil pumped from the supply pump 28 and the scavenge pump 18. The recirculation valve 23 also has a first output 36 through which the minimum critical supply of oil flows to the integrated drive generator 12. As stated above, the oil flow from the first output 36 must always be above a minimum oil flow rate in order to avoid serious damage or catastrophic failure of the integrated drive generator 12. The recirculation valve 23 also has a second output 38 which applies oil to the input 26 of the aircraft mounted accessory drive 24. Flow path 40 indicates the operation of the recirculation valve to recirculate oil under normal operation from the output 22 of the scavenge pump 18 through the second output 38 to the input 26 of the aircraft mounted accessory drive 24. In this circumstance, all of the oil outputted from the aircraft mounted accessory drive is pumped by the head produced by supply pump 28 through the input 32 of the recirculation valve 23 and to the first output 36 of the recirculation valve to the input 14 of the integrated drive generator 12 to satisfy the minimum oil flow requirement. As will be described in more detail below, assuming that the quantity of oil provided by the flow from the aircraft mounted accessory drive 24 through the input 32 is interrupted, oil is shunted from the input 34 of the recirculation valve 23 under the head produced by scavenge pump 18 to the first output 36 of the recirculation valve 23 to the input 14 which, in all instances, is desirable and if not accomplished can cause undue wear or damage. The foregoing operational modes are described in more detail in FIGS. 2-4 which illustrate specific operational modes of the oil circulation system 10 of FIG. 1.
FIG. 2 illustrates a normal mode of operation of the recirculation valve 23 when all of the minimum oil flow requirement of the integrated drive generator 12 is satisfied from oil flowing from the aircraft mounted accessory drive 24 through the first input 32 to the first output 36. The recirculation valve 23 has a valve stem 50 which moves within valve body 52 between a first position and a second position with the second position being illustrated in FIG. 2. The valve body is biased by a spring 54 to the first position as illustrated in FIG. 3. The pressure of the pumped oil produced by the supply pump 28 is applied to a first face 56 of the valve stem and the pressure of the oil pumped by the scavenge pump 18 is applied to a second face 58 and a third face 58'. As is apparent, the relative surface areas of the third face 58' and the second face 58 are such that they apply no net force to the valve stem 50. The valve stem position is unaffected by pressure produced from the scavenge pump 18 and is determined by the bias spring 54 and the pressure produced by the supply pump 28. The oil flow paths 60 and 62, respectively illustrate the flow of oil between the first input 32 and the first output 36 and the second input 34 and the second output 38.
The recirculation valve 23 has a first port 62' in fluid communication with the first input 32 for receiving oil from the first input 32. Oil flows through the first port 62 into a first chamber 64 and out through a second port 66 in fluid communication with the first output 36. Oil flows from the second input 34 through a first port 68 in fluid communication with a second chamber 70 out through a second port 72 in fluid communication with the second output 38.
FIG. 3 illustrates the second mode of operation in which the minimum oil flow to the input 14 of the integrated drive generator 12 is satisfied totally from oil flowing from the output 22 of the scavenge pump 18 through the second input 34 out through port 100 to the first output 36 to the input 14 of the integrated drive generator 12. In this particular mode of operation, the force exerted by the spring 54 biases the valve stem 50 to the first position in which the face 56 of the valve stem is seated against the first port 62 to block the flow of oil 60 from the input 32 to the output 36. In this particular mode of operation, the pressure of the oil flow 60 outputted from the supply pump 28 drops sufficiently below the pressure required to overcome the force exerted on the valve stem 50 by the bias spring 54.
The first mode of operation, as illustrated in FIG. 2, is the ideal mode of operation in which the required minimum oil supply is from the aircraft mounted accessory drive 24 where it is cooled, filtered and routed to the inlet 14 of the integrated drive generator 12 through the first output 36 of the recirculation valve 23. FIG. 3 illustrates the alternative methodology for supplying the minimum flow to the inlet 14 of the integrated drive generator 12 in the absence of an oil supply from the aircraft mounted accessory drive 24 which is made up by the oil flow 62 outputted by the scavenge pump 18 pumping oil from the integrated drive generator 12. The modes of operation depicted in FIGS. 2 and 3 do not pose a problem of operation.
FIG. 4 illustrates an operational mode of transition between the fully open position of FIG. 2 in which all oil of the minimum oil flow to the input 14 of the integrated drive generator 12 from the first output 36 of the recirculation valve 23 is satisfied by oil pumped from the aircraft mounted accessory drive 24 through the input 32 and the closed position of FIG. 3, in which all oil of the minimum flow pumped to the input 14 of the integrated drive generator 12 from the first output 36 of the recirculation valve 23 is satisfied by oil pumped from the scavenge pump 22 through the input 34. In the transition mode, the position of the valve stem 50 between the first position as illustrated in FIG. 3 and the second position as illustrated in FIG. 2, respectively, is controlled solely by the pressure of the oil flow 60 at the first input 32 and is insensitive to the output pressure of the oil flow 62 from the scavenge pump 18. When the recirculation valve 23 is operating in the fully open position as illustrated in FIG. 2, and the pressure of the oil flow 60 drops below that required to exert sufficient force on the face 56 of the valve stem 50 to overcome the force exerted by the spring 54 at the fully open position, the valve stem moves to the left as illustrated in FIG. 4. This creates an additional drop in the oil pressure at the inlet 14 of the integrated drive generator 12 because the flow path through the valve 23 from the inlet 34 and the inlet 32 through the outlet 38 to the reservoir 30 of the aircraft mounted accessory drive experiences substantially less flow resistance to the reservoir 30 than the flow resistance to enter the integrated drive generator 12 through outlet 36. This produces a distinct drop in the net flow to the integrated drive generator 12 through the input 14 through the first output 36 thereby partially starving the integrated drive generator of the requisite oil supply below the required minimum flow rate. The valve stem 50 dwells in the intermediate position until either the force from the oil flow 60 applied to the face 56 drops below the force exerted by the spring 54 at the valve closed position of FIG. 3 or, where the flow to the reservoir 30, a combination of flow from the scavenge pump 18 and the flow from the aircraft mounted accessory drive 24 is large enough to generate a back pressure to the supply 28 sufficient to raise the pressure applied to the face 56 to compress the spring 54 to restore it to the fully open position as illustrated in FIG. 2. It takes a relatively small change (increase or decrease) in the oil flow 60 through the inlet 32 to move the valve stem 50 into the transition range, but a substantially larger change in flow 60 to achieve transition to either the fully closed position or the fully open position.
In actual practice, operation of an integrated drive generator can occur below a rated pressure, such as 100 psi minimum, but above a pressure which is indicative of an oil interruption mode of operation. This mode of operation corresponds to that illustrated in FIG. 4 in which the overall flow and pressure of oil to the input 14 is representative of a starvation condition below the minimum required flow rate. It is necessary to avoid damage to integrated drive generators to prevent the operational mode of the transitional nature, as illustrated in FIG. 4, because of a significant possibility of increased wear or failure resultant from starvation of the integrated drive generator parts from the condition below the minimum oil pressure flow, such as the minimum 100 psi pressure, required on some integrated drive generators manufactured by the Assignee of the present invention.